Sex differences are widespread in both mouse and human liver, and are associated with clinically relevant sex differences in hepatic drug and steroid metabolism, lipid metabolic profiles, the incidence of liver cancer, and cardiovascular disease risk. This application focuses on the genomic and epigenomic actions of growth hormone (GH), a pituitary polypeptide hormone and major regulator of liver metabolic functions, in particular those that show sex differences. The studies proposed test the hypothesis that GH regulates the chromatin states of sex-specific genes, and thereby establishes an epigenomic environment that facilitates the sex- specific actions of GH in the liver. The mouse model will be used to investigate the mechanisms by which GH, via its sex-specific temporal patterns of pituitary gland secretion (pulsatile in males vs. near continuous in females), activates transcriptional pathways and epigenetic events that regulate hundreds of liver-expressed genes in a sex-specific manner. Major progress during the last project period included the discovery of key GH-dependent transcriptional activators and repressors that interact on a genome-wide level and regulate sex differences in the liver, the development of global maps of accessible chromatin regions (DNase hypersensitivity) and epigenetic signatures (chromatin marks) that identify several hundred sex-specific regulatory elements, and the discovery of novel sex-specific long, intergenic non-coding RNA genes that are GH-regulated and may determine sex-specific chromatin states. These findings provide a unique opportunity to move the field forward by elucidating genome-wide, and at the epigenetic level, the fundamental biological mechanisms that underlie the complex regulation of sex differences in the liver by pituitary GH secretory patterns. This will be accomplished through the discovery of: 1) chromatin states and their associated transcription factor motifs that characterize distinct subsets of sex-specific genes; 2) distal regulatory elements and their interactions with sex-specific target genes; and 3) the role of sex-specific, GH-regulated long, intergenic non-coding RNA genes in establishing and maintaining the sex-differentiated chromatin states that facilitate sex differential liver gene transcription. Together, these studies will identify key mechanistic features that determine the complex, GH-regulated and sex-biased expression of genes that control liver metabolic processes with a major impact on human health. The results obtained are expected to have a high impact, shifting the mechanistic focus of studies on GH action to the epigenome, and will serve as a paradigm for other endocrine factors that alter the epigenome in complex ways.